Carrier-wave transmission



April 1o, 192s. 1,665,328

H. R. SCHMITT CARRIER WAVE TRANSMISSION April 1o, 192s. 1,665,328

H. R. SCHMITT CARRIER WAVE TRANSMISSION Filed May 12. 1923 3Sheets-Sheet 2 ofo.

April 1o,- 192s. 1,665,328

H. R. SCHMITT CARRIER WAVE TRANSMISSION Filed May 12, 1923 3Sheets-Sheet 3 f/y v v tieni.

Patented Apr. 10, 1928.

untreu sr-Arssrarrur omer..

HUGO R. SCHMITT, NEW YORK, N. Y., ASSIGNOR T0 WESTERN ELECTRIC COMPANY,INCORPORATED, 0F NEW YORK, N. Y., A CORPORATION Oli' NEW YORK.

Giannina-WAVE 'rnansnrssron application sied may 1 2, 1923, serial No.scarsi. i

of radio signals;

object of the. inventionis to provide Arath@ commenaiensystems which@0mbini t0 'rester degPee'then-pribr. Systems, eliicient utilization ofelectric energy, economy. 'Qt frequenta rees-e andA Secrecy.;

A, feature' of the. invention, rela-testo an iepreved-met-hede ,Sialias; with' a lsingle Sttel .Sdefbeed' {v'thet .the enmodlated Wam@perusing atlzne the, local transmit. iPg'SPwTCe t0. irovide it. .eCarrier freqneni 'iiwhidh ,necessary for; lieen .9. giveanjesil'dedjmeasure greenery, thev fteqneilees f, the. carriers.ltortue-.directin transmission-ina synchrononsly variad@ @sier.Iripfeaiya printneteleemrh .apparafiisfat t erespeetve Stations, or thesans! 'bend may befinvetedln ireqilncy synchronised variations similarlycaused to occur inthe inverted Waves.v

Another feature relates to the transmission of asn pressed carrier,signal modulated, wave 1n..w hich the carrier frequency isconti nuously4varied at one or both of the stations, asby means of printing telegrapha patatas, in which the Acarrier Wave is reproduced at the receivingstation without the use of local sources.

Additional features are an efiicient. arrangement for impressing thecarrier Wave on the grid of anelectric discharge modulator and acompensating means in the output circuit of thev modulator-'to maintainthe desired squaredcharacteristic under operational conditions.

Communication by use of single side band,

suppressed carrier, transmissionvhas the ele-- ment of secrecy inherentin suppressed 'carrier systems generally, and in 'addition theadvantages: of relatively great economy of power and of frequencyrange.y The use of at least two stages of amplification at thetransmitter is desirablein order to eiect the complete separation of oneside band from the other components. Applicant proposes to utilize`one'of the localsources required for this function to supply the in-Acoming carrierl for'reception. If the difference' frequency' method ofreceiving is @este eem satien ofthe Wares 'rem the two local sources maybe used to supply the transmitted carrier at the receiver.

' Ina modified form of the single side band suppressed carriertransmission system, designed to increase the degree of secrecy, thefrequencies of the unmodulated carrier componente are synchronouslyvaried atl the tivo stations. This may be accomplished l by mea-ns of aprinting telegraph apparatus, Vwhich may also supply a secret telegraphchannel. Since synchronization insures the continuance of the initialfrequency rela- .'t n between. the, OPPO'Sltelydrted Waves,

eveil. when the fieqlle'ny swflied the mode reception above' described,in `which the lgaln"tran smitter sources are utiliztedfor re-;Ceptwamaybe used- A' Somewhat Similar secrecy system is provided byinvertingl the frequeneesfof the sseal'vave and. Synchro- OuSlyVar-'ying the frequencies. 0f' the iriverted Waves. at ,the tiveStations by means 0f 'the Printiveteleararh apparatus.-

lIn van alternative secrecy system characterized by suppressed carriertransmission and in which the carrier is continuously varied,synchronisation of the variations at the tivo stations is notvattempted, but the carrier is reproduced at the receiver by derivationfrom a. minute amount of carrier which is independently', transmittedfor that purpose. rlhe difference frequency method ot reception is usedto obtain a separable frequency component derived Jfrom said carrier`which may be amplified and combined with the minute portions of theincoming carrier to produce the carrier in sufficient amount to be usedfor detection.

An improved form of balanced modulator permits the carrier wave to bcmore direct-ly impressed on the input electrodes thus avoiding shuntingthrough the. distributed capacity of the low frequency transformer.

A further improvement in modulator circuits relates to the use of acompensating tube having characteristics similar to those of themodulator tube or tubes, in the space Current path of such modulator.The impedance of this tube varies with that of the modulator tubes. Thisinsures that the potential impressed on the plates of the modulatorremains constant during modulation.

The invention may now be more fully understood from the followingdetailed clescription when read in connection with the accompanyingdrawing, Figs. 1, 2, 6, 7 and 8 of which constitute circuit diagrams oftwo-way radio communication stations embodying the invention, Fig. 3 ofwhich i1- lustrates a detail of the system of Fig. 2, Figs. 4 and 5 ofwhich, together with Fig. 2, constitute another modification of theinvention, Fig. 9 illustrates a modulator circuit adapted to be usedwith any one of the modifications of the invention, Fig. 10 illustratesa further improvement in modulator circuits and Fig. l1 illustrates acircuit which may be substituted for the balanced modulator and itsassociated selective circuit in any one of the signaling systemsdisclosed. In these figures, reference characters which suggest thefunctions of the respective element-s are used. For example, A, BAL M,BF LF and HO will be used to indicate respectively an amplifier,balanced modulator, band filter, low pass filter and a high frequencyvoscillator. l

Fig. 1 discloses the circuits Vused at each station of a differencefrequency two-way radio communication system which isvcharacterized bysuppression of the unmodulated carrier wave component and single sideband transmission. The circuits at the left of antenna AN are used fortransmission, those at the right for reception. The operation of thetransmitting ycircuit is as follows:

Speech or other signal Waves from source 1 are limitedby low pass filterLF to their essential frequency range and impressed together with a wavefrom intermediate frequency oscillator or oscillator-amplifier IO onbalanced modulator BAL M. The resulting modulated intermediate frequencycarrier wave is transmitted by this modulater free from any unmodulatedcarrier frequency component to the band filter BF, which selects asingle side band. It will be assumed that the upper side band is the oneselected, although either side band may be selected. The label f-l-Sindicates that this side band has a frequency which is equal to the sumof the intermediate carrier frequency ,f and that of the signal S. Thisside band is used to modulate a wave from high frequency oscillator HOin a second balanced modulator BAL M1. A single side band of themodulated wave, comprising, in the case assumed, frequencies ofF-I-f-l-S, is selected by band filter BF1, amplified by power amplifierPA, and radiated from antenna AN. This wave is in effect the upper sideband of a radio frequency carrier wave, having the frequency F-l-f. Thisradio carrier frequency will be designated C. The received wave isSimilar in 'characteristic to this transmitted wave and may be similarlyrepresented as comprising the radio carrier C1 modulated by a signal S1.Assuming, for the moment, that waves of frequencies C and C1 aresupplied respectively by circuits 2 and 3, the received wave 'C14-S, isdetected and the signal indicated by t-he following sequence of steps:

Detection by high frequency detector HD yields a difference, orintermediate, frequency C-C, and the signalA modulated differencefrequency C-C,-S,; these. waves are amplified' by intermediate frequencyamplifier IA and detected in intermediate frequency detector ID, whichyields among other components a signal S1, which, after lapproximate thefrequency of the desired wave thatthey could not 'be separated therefromby antenna tuning." y,In this ease` detectionhin .one stage 'would l,result inthe superposition of two orfmo're signalwaves in the output ofdetector'HD.

An important feature of the linvention is a means for obtaining ,the.two radi Carrier waves required fory receptionQwithout recourse to the`use of local sources other than thosel necessarily requiredfortransmission from the same station. The radio carrier wave of frequencyC is obtained by combining the waves from the two local sources IO andHO in modulator M and selecting the upper side band F- l-f by bandfilter BF2. The radio carrier C1 is obtained directly from source I-IO,the frequencies ofthe in-y termediate and high frequency carrier .wavesmaking up the radio carrier'wavev transmitted from the distant stationbeing adjusted accordingly. The necessary relation of frequencies, inorder that the above de-v scribed operation may be performed identicallyat the two stations, will be satisfied, if the following frequencies areused at the respective stations: the wave transmitted from stationA=C+S=Flf+S; t-he wave transmitted from station B=C1lS1=F,L ffl-S1F=C1=F1f1; F1=C=F+f f=f1. .It should be noted that the use of ldentiealeircuit arrangements at both stations requires that after each stage ofmodulation side bands of opposite sign be used for the two stations.

The system described is characterized by relatively great economy ofpower, economy of frequency range, a minimum of elements for two-wayoperation with single side band transmission, and secrecy to the extentthat an interceptor must have a properly adjusted local, sourceA inorder to obtain intelligible reception.

Fig. 2 illustrates an improvement over the system of Fig. 1characterized by greater secrecy. This results from the use of avariable high frequency carrier. It is not n cc es sary to. use singleside band transmission or successive modulation, although these featuresaredesirable. If successive modulationis used, the frequency. 'variationofthe radio carrier Wave may vb'e secured by variations in the firstinstance in the wave from either the intermediate or high frequencycarrier source. The frequency variation canbe accomplished very simplyby the means diselosed in Fig. 3- in which there is shown an'oscillator, 'the frequency of which is determined by the constants of a4tuned circuit. This'circuitv includes a condenser varied periodicallyas disclosed in Canadian patent'y to' Beatty Serial No. 197',- 181,patented, February 10, 1920'.

For the most complete measure of secrecy, the variation maybe'cause'd'to occur at each of the two stationg'although, 'of course,the advantages of this system"may be realized in partif thevariation'occurs at one station only. AThe receiving 'circuit would bethe same for'botlr'` ":a'sfes' .'"`l

l'lhe circuit 'arrangement disclosed in Fig. 1 for obtainingthe'rinteracting frequency .Wave for rece tion cannot beused in thissystem, since t e frequencyE relation between the4 vin con'iing andv"outgoing frequencies,

Whichis a necessary feature of such system,

would require synchronous variations at the tivo stations. 'In order toobtain this interacting frequency 'Wave a very minute amount ofunmodulateda'adio carrier is transmitted by combining a small .portionof the outputs of thetwo oscillators HO.- and IO in. modulator M,selecting the upper side band by band filter BF, impressing it on theinput of the power amplifier PA. This unmodulated component isinsuiiicient in amount to permit signal detection by any ordinary means.It would be practically impossible to se arate it from its side bands atthe receiver for the purpose of amplifying it sufficiently to permit ofnormal de- '.tection. Detection in 'high frequency detector HD gives aminute: amount ofl difference frequency carrier Wave. This differencefrcquency C--C1 may be readily separated from its signal side bands,amplified in ainplilier A and impressed o n the input of the detectorHD. 'lhe combination of this amplified difference frequency with theminute incoming radio carrier' C in the detector gives sufficientamounts ofthe radio carrier C. vfor detection.- Dteetiqn is aeemplshedby detecter. HD1 after emrlietiea. if d- Slireitby 11i .11. .freelweyamplifie' HA- The' detecter. sie? 'beemiiliedf by speech amplifier SAand indicatedl by telephone receiver 4. i i

The system of Fig. 2 modified by the substitution of the circuits of F lfor the circuits shown at the left of line X-X, and by the substitutionof the circuits of Fig. 5 for the circuits shown at the right of lineY--Y, provides for all of the functions of the system of Fig. 2 and anadditional printing telegraph channel.

The speech frequency band. in this moditica-tion of Fig. 2, is limitedby band filter BF 3 to frequencies above a lower limit high enough topermit the use of a telegraph fre quency. For example, the filter maypass frequencies between 200- and 200,0 cycles. Low frequencyoscillator' LO and low pass filter'. LF, supply a current of a lowfrequency, for example 100 cycles, for printing telegraph transmissionand for synchronizing the rotating par'ts of the printing telegraphtransmitter and receiver at the r' espective stations. 'This current isapplied to the printing telegraph transmitter and synchronizingapparatus P. The telegraphl frequency output current is impressedtogether with speech current and the current from source 10,.on thebalancedl modulator BAL M. The frequency of the current from the highfrequency oscillator HO is varied synchronously from the moving parts ofthe printing telegraph apparatus in any well known manner. As in thesystem of Fig. 9., the variations may be caused to occur' in thefrequency of either oscillator HO;

as disclosed, or of the oscillator IO. By using an irregular shapedcondenser plate as e. g. that disclosed in Canadian patent to BeattySerial No. 197,181, patented February 10, 1920, a complex variation offrequency can be produced which would be eX- ceedingly difficult for aninterceptor t0 reproduce in his local oscillator'.

A type of printing telegraph apparatus that may be used, with thechanges incident to operation in a radio communication system, isdescribed in a paper by John H. Bell, printed in the Transactions of theAmerican Institute of Electrical Engineers, volume 39, part- 1, page166, in which see the subject matter under Multiplex system, page 193.The system there shown is adapted for the usc herein proposed by' merelyreplacing the direct current synchronizing and signaling sources byalternating current source LO and by completing the circuit from thissource through the printing telegraph transmitter at the transmittingstation' so that the telegraph currents may be utilized as modulatingcurrents in the manner customary in radio transmission c ircuits. Ifdesired, different frequencies may beused for synchronizing and fortelegraph einem The detected low frequency currents at line Y-Y of themodified receiver of Fig. 2 contain components corresponding to speechand the telegraph currents delivered from the output circuit ofapparatus P. These are separated by band filter BF, and low pass filterLF2. The speech is indicated by telephone receivers 4 and the telegraphand synchronizing currents are impressed on the printing telegraphapparatus l?1 which comprises the usual synchronizing and recordingapparatus for a printing telegraph receiver. The use of a printingtelegraph apparatus makes possible simultaneous secret transmission oftelephone and telegraph signals since the telegraphic message is in theprinting code and both the telephonie and telegraphic messages aretransmitted by a varying frequeney carrier Wave.

The use of a synchronizing means insures a constant dierence between thecarrier frequencies of the waves transmitted in the two directions.Accordingly, the' carrier Wave may be vsupplied by'- derivation from thetransmitter sources asin the system of Fig. 1 instead of by thearrangement disclosed in Fig. 2. Circuits for accomplishing this areillustrated inFigs. 64 and 7. The transmitting circuit of Fig. 6 isthesame as that -of Figs. 2 and 4 except lthat the radio carrier lfrequency is varied by varying the frequency of the intermediate,insteadof the high frequency, source. The receiving circuit is like thatof Fig. 1 but instead of a difference frequency receiving circuit simpledetection is used, the frequencies being the same for transmission inboth directions. If the difference frequency receiving circuit Wereused, the Wave from the high frequency oscillator and the combined Wavefrom the two oscillators could similarly be used to supply respectivelythe incoming and outgoing radio carriers. As in thesystem of Fig. 2, thefrequency variations may be caused to occur in the Wave from either ofthe two oscillators.

Fig. 7 illustrates a system which is the same as that of Fig. 6 exceptthat a single stage of modulation is used. Elements similarly designatedhave the same functions as the corresponding elements in Fig. 6. Sincethe use of modulator M is not necessary the amplification incidentallyafforded thereby may be secured if necessary by amplifier A1. On accountof the difficulty in effecting separation of the side bands at the highfrequency used for radio transmission, sucli separation Would in generalbe less perfect than if the system of Fig. 6 were used. The circuit ofFig. 7 could be effectively used if it were not attempted to obtain theadvantages incident to single side band transmission, but if both sidebands were transmitted. This could be accomplished by merely eliminatingthe filter BF1 or by substituting therefor a filter which passedsubstantially both side bands.

Fig. S illustrates the low frequency circuits at a two-way radiocommunication station in which secrecy is obtained in part by speechinversion and in part by variation of the frequency of the invertedspeech, this variation resulting from a variation of the frequencyof theinverting source. A printing telegraph apparatus is used to producesynchronized variations of the inverting frequency at the transmitterand of the reinverting frequency at the receiver in addition to itsnormal function as a part of a printing telegraph channel. The speechinversion system operates according to the method disclosed in BritishPatent N0.

vcommon modulators for the transmitting and receiving functions of eachstation.

Speech from microphone 1 or the like is limited by low pass filter LF3as in the systems of Figs. 6 and 7 in order to provide lspace for thesynchronizing and telegraph frequencles.

The resulting speech band is impressed with current from oscillator O onthe balanced modulator BAL M2 Which, together with band filter' BFS,eliminates all the frequency components except the upper side band, thefrequency of which may be of the order of 9000 to 11,000 cycles. Insteadof a balanced modulator, a simple 4modulator may be used, reliance beinghad on the filter to eliminate the carrier frequency. This side band isimpressed with current from oscillator O1 on the second balancedmodulator B. L M3. The frequency of oscillator O1 is varied by theprinting telegraph apparatus, as in the previously described systems.Vith the assumed value of the modulated side band the current fromoscillator O1 may be given a frequency varying from 11,100 to 11,600.The lower side band in effect an inverted speech band whichvaries fromsubstantially its normal frequency range 100 to 2100 cycles to the rangefrom 600 to 2600 cycles. The preliminary stepping up by the. modulatorBAL M., before inversion is for the purpose of providing a more completeseparation of the band to be inverted and accordingly to avoid thepresence of interfering components Which would otherwise result from theinverting operation. This variable inverted speech is selected by bandfilter BF6 and utilized, with the telegraph tone and synchroxiiz'ingcurrent, in subsequent steps of modulation and' radiation, which stepsmay conform to any of the conventional usages of radio transmission, asillustrated in part, for example, in Figs. 1 and 2.

Vaves incoming from the radio receiving circuit, which may, as in thecase of the transmitting circuit, conform to any of the conventionalusages of radio reception, as illustrated, for example, in Figs. l and2, are separated into its inverted speech and telegraph note componentsby band {ilter BF, and low pass lilter LF4. The latter is impressed' onthe printing telegraph synchronizer and recorder P1. The inverted speechis stepped up by combination with thecurrent .from oscillator O1 inbalanced modulator BAL M3, the upper side band being'is'elected by bandfilter BFB which is identicalinall; respectsv to band .filter BF5. Reinverted-'speech is obtainedV from this side band. by combination with acurrent from oscillator O in balanced modulator BAL M2. Theresultantcurrent contains current of the telegraph note. as well asofspeech.Speech current isz-separated by the band filter BF, and indicated in.telephone receivers 4. 4The..-pr'intin'g'telegraph apparatus at onecircuiti-'maybe considered as a master controlling means, the othercircuits at the twollsiiations both transmitting 'and receiving,beingpnovided .with means for keeping their printing telegraphvapparatus in synchronismtheifewith. .Accordingly, it isimmaterialfasztolwhich telegraph apparatus is used at eachA station tocause. the oscillator frequencyfvariations. As a matter of practice,the' rotating parts ofthe printing telegraph apparatus for transmittingand receiving at each station are usually mechanically coupled. l

` Fig.' 9- illustrates. the circuits of a novel form of modulator whichmay be used in any' one of the systems of the invention. The -modulatoris of the balanced type.

In the specific circuit arrangement of the figure, the tubes 5 and 6 arealike and of the usual three-electrode type. These tubes are related asin the balanced modulators disclosed in Fig. 3 of Espenschied Patent No.1,447,204, issued March 6, 1923, except for the arrangement of thecarrier Wave input circuit. The circuits andtheory of operation of thisgeneric type of balanced modulator' are described in Carson Patent No.1,343,- 307, issued June 15, 1920, to which reference is made for a moredetailed disclosure.

In' vacuum tube circuits generally it has becomey the practice to groundone of the terminals of a 'cathode as at 7 in order to pre- Ventundesirable fluctuations in the capacity ,between the parts of thecircuit and ground. It' has been -ou'nd that, when using the if-thecarrier. current is impressed on the input electrodes of the tubes.by'means of a connection to the common input lead, the capacity betweenthe modulating Current input coil and ground, and other capacities,either intentionally used or unavoidably present between the inputelectrodes and ground, tends to shunt the carrier current from the inputelectrodes, necessitating a greater amount of input energy to maintainthe input electrodes at the desired potential; It also results from theuse of this connection that there is an undesired variationv of suchpotential. This shunting effect increases with the frequency of thecarrier currents so that in certain radio frequency cir-4 cuits, asinthe high frequency balanced modulators elsewhere disclosed in thisapplication, an abnormal amount of input energy is required.

This effect is avoided in the present cir cuit by connecting, thecarrier` input circuitsdirectly to the' inputl electrodes through largecapacity condensers 9. andi()v which are large enough to constitutesmall imped? ances to the carrier frequency but enough to preventany-appl'B Gaglille.y lovr of modulating current .inv this circuit.i'lilley shunting effect isaccordjng-ly avoided the input electrodesreceive the full variaf tion of potential which exists in the secondary'of transformer 11 unaffected by the elec# trical condition ofthemodulating. or other circuits. Itis desirableY although not 6.85.311-

tial to provide highfrequency choke 12 and 13 topreven'tthecarrerlcurrent ,from

flowing in the speech input circuit; Theusev of separate input circuitsfor the carrier and modulated frequencies, with these condensers andchoke coils, insures a complete independence of function of these.circuits. It should be noted that although the invention has beenembodied in a balanced modulator circuit, it may be. similarlyembodiedin a simple modulator having-a grounded cathode.

Fig. 10 illustrates a further improvement of a balanced modulatorcircuit. The improvement consists in the addition of the compensatingtube 14 and the circuits associated therewith to a balanced modulatorcircuit.

To effectively suppress transmission of th unmodulated carrier componenta balanced modulator requires that the characteristic curves of the twomodulator tubes, during modulation, be identical. The condition of amodulating tube space current circuit during modulation is similar tothat of a variable iinpedance in series with a fixed impedance across aconstant potential source. The potential at the junction of the twoimpedances varies as the resistance varies to constitute a Variablefraction ofthe .whole imp edance. Applicant proposes toy cause a coren lthe amplified wave and thus be additive to responding variation of theotherwise constant external 'im edance and accordingly to keep the ratioofp the internal and external impedance's, and therefore the potentialon the plates, constant. This permits tubes which are balanced under'static conditions to be balanced under working or modulating conditions;

Compensating tube 14 has characteristics similar to those'of modulatortubes 15 and 16. Its input circuitis subjected to similar variations ofpotential by circuit 17. Its internal plate-filament circuit accordinglyhas corresponding variations of impedance. Since the impedance of thisinternal platefilament circuit'is in series :with the correspondingVimpedances of the. modulating tubes, it results that 'the'division ofpotential drop between the internal plate-filament circuits of "themodulator fand the remaining part ofthesp'ace current circuit thereof,in-

cluding the` 'compensating tube, remains constan't."v.Thepotentials-impressed y'on the platesf- =ofthe -modulatortube'saccordingly remain substantiallyfconstant. 1 'l"{ljliecompensating *tube I may be used to compensatetffortheleffectoflthe resistance of th-'space "current source fitself if thissource is"'directly connected tof-the jplatesof the modi'ilator.4 ,As-u'sed-inthe'circu'it of Fig. 10', it 'would compensate. for'the effectof that resistance-plus the f resistance of thel output coils.1`Itis''especially useful to replace a comparatively' large loadingresistance which lill wouldl otherwise-have to be lused in order to makepossible the use of aspace current source f'lhigher potential than thatfor which the tubes are adapted. In this last case as well as where themodulator output circuit contains yconsiderable resistance in additionto that'furnishe'd by the compensating tube, the constants yof this tubewould be chosen so thatthe variations in its internalplate-filamentimpedance are somewhat greater than the correspondingvariations of the modulator'.

Although particularly useful in a balanced modulator, as explained, thecompensator may be similarly used in a single tube modulator to preventthe distortion which would otherwise result from the flattening of thecharacteristic curve during operation, and Fig. 10 is equally intendedto illustrate this use. This result is secured by the operation of thecompensa-ting tube in maintaining the directcurrent plate potentialconstant. The same-function may be accomplished in an amplifier tubearrangement as by emitting the carrier frequency circuit 18. Its use inthis circuit would result in less distortion of that ordinarily obtainedwith'balanced amplifiers.; Of course the compensating tube would have anequally important function in a single tube amplifier.

Although the use of a balanced modulator i followed by a selectivecircuit has been illustrated in each of the gures which disclose singleside band suppressed carrier transmission, reliance may be had on aselective circuit alone to eliminate the carrier and one side band. Fig.11 illustrates one-circuit for accomplishing this function. M1 mayvbeany modulator adapted to produce -a modulated carrier wave comprising anunmodulated carrier component and the upper and lower modulation sidebands. Circuits A19 and 20 are tuned respectively to one side band andthe unmodulated carrier frequency and effectually eliminate thesecomponents from the output of the-filters. Circuit 21 is tuned to thedesired sideband and accord'- ingly resonantly selects and accentuatesthis component. This filter circuityis especially adaptable toamodulating system .in.which the modulating wave has either asingle-frequency 4or comprises a relativelyl narrow.

band-of frequencies.

\ Itwill be understood the circuits and methodsv herein set-.forth maybe embodied in organizations widely different from .thoseillustratedfwithout 5de: parting -fromthe spirit iof 1 `the a inventioncathode and control electrode, whereby any shunting to ground of either'.wave at 'a point anterior to said control electrode cannot affect thepotential impressed thereon.

2. A translating device having an element substantially at groundpotential for high frequency currents, input electrodes for said deviceincluding said element, means to impress on said input electrodes highfrequency potentials, means to impress thereon low frequency potentials,and means to prevent the dissipation of the high frequency currentthrough the capacity to ground of said low frequency impressing means'from affecting the values of said impressed high frequency potentials.

3. The combination of an electronic valve modulator having a groundedheated cathode, an anode, and a control electrode, .a modulating circuitconnected to the cathode and control electrode, said modulating circuithaving a capacity, to ground in parallel with the control electrode, asource of current of high frequency and means connecting said source tothe cathode and control electrodes in such manner as to prevent saidcapacity from shunting the carrier currents from the control electrodes.

4. A modulator comprising in combination an electron discharge devicehaving a grounded cathode., an anode and a control electrode, an inputcircuit connected to the cathode and control electrode, means to couplea source of modulations to said input circuit, a second input circuitconnected to the cathode and control electrode, means for coupling asource of high frequency oscillations to said second input circuit, andmeans whereby currents of the modulating and high frequencies are eachconfined to their respective individual circuits.

5. The combination of a modulator having two electronic valves eachhaving a grounded heated cathode, an anode, and a control electrode,parallel input circuits for said valves including means connecting thecathodes and individual paths connecting said cathodes and theindividual control electrodes, a source of modulating signal currentvariations associated with the individual paths, a direct high frequencyconnection between the control electrodes of said valves, and a sourceof high frequency oscillations connected between said control electrodeconnecting circuit and the cathodes,

said individual circuits and control electrode connecting circuitincluding respectively high frequency choke coils and blockingcondensers, whereby said sources can, each independently of the other,impress its variations on the control electrodes.

6. A modulating system comprising a pair of translating devices havinginput and output electrodes, a circuit containing a source of highfrequency energy, a circuit containing a source of low frequencymodulating energy, means for symmetrically associating said highfrequency circuit with said input electrodes, means for oppositelyassociating said low frequency circuit with said input electrodes, andan output circuit differentially associated with said output electrodecircuits, said high frequency associating means comprising a circuitdirectly connected to said input circuits in parallel and includingblockin condensers to prevent the current from mo ulating source fromflowing therethrough.

In witness whereof, I hereunto subscribe my name this 11th day of May A.D., 1923.

HUGO R. SCHMITT.

